The 4th shouldnt be reachable till further into the game, sorry that the puzzles arent more clear :( hope you go back to it!
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There are open books on the bookshelf next to the keypad, they have symbols on them like a dot, | , ; and C, then there are 4 bookshelves scattered around that tell you what these symbols correspond to and you just insert the numbers that correspond to the signs on the books as the code, then you can reach the fourth sign!